The present invention relates to a manufacturing process for a printed wiring board. Particularly, the present invention relates to a manufacturing process for a printed wiring board performing drilling by carbon dioxide laser.
Precise drilling has been performed, heretofore, to form a through hole (PHT) in a printed wiring board, but because of today""s built-up demands for high density circuit formation and high density mountability on a printed wiring board, microfabication has become required for a through hole or a hole such as IVH, BVH, a marker hole or the like in a field of a multilayer printed wiring board with three layers or more.
With such a change in requirements of the market for a printed wiring board, a drilling technique using laser light such as of carbon dioxide gas laser, YAG laser or the like has been developed as an alternative to the drilling technique.
On a surface of a copper clad laminate for use in manufacture of a printed wiring board, however, a copper foil forming a conductor circuit is present and a surface of the cooper foil is called a shiny side and has an arithmetic mean roughness (Ra) of the range from 0.01 to 0.3 xcexcm; therefore, the surface thereof is mirror-finished in a similar level to that polished. Consequently, even when the copper foil surface is irradiated with carbon dioxide gas laser light, a reflectance of the laser light thereby is high; thus having resulted in not only impossibility of machining to a desired shape on the copper clad laminate with carbon laser light in a short time, in the presence of a copper foil at a surface of a copper clad laminate, but also difficulty in application of the machining to actual operation.
Therefore, laser machining applied on a copper clad laminate has been adopted in the case where carbon laser light is applied in conformal mask lithography in which only portions of a copper foil thereon where a through hole or a hole such as IVH, BVH, a marker hole is drilled are removed by etching in advance, followed by laser irradiation drilling.
The conformal mask lithography process, however, has a fault that since portions of a copper foil at drilling positions are removed by etching prior to laser drilling, extra steps otherwise unnecessary are added, such as etching resist formation, exposure, developing, copper etching, etching resist removal and others, having resulted in a factor to raise a manufacturing cost for a printed wiring board.
In the case where carbon dioxide gas laser is adopted, studies have been conducted, in order to solve the fault of the conformal mask lithography process, on a technique enhancing an absorbance of laser light on a surface of a copper foil of a copper clad laminate to remove a copper foil and resin as a substrate material simultaneously in a short time, having tried various other measures, that is, having applied so-called black oride treatment on the surface of a copper foil of a copper clad laminate, or having provided a resin layer with high laser light absorption efficiency thereon with an intention to enhance laser light absorption efficiency on the surface of a copper foil of a copper clad laminate.
Assume the case where the so-called black oring treatment is applied on the surface of a copper clad laminate. However, the black oring treatment, in essence, is a process performed after a through hole or a hole such as IVH, BVH or the like is formed and then interlayer connection copper plating is performed, followed by forming a circuit through etching, and is to be applied on an internal substrate material of a multilayer substrate. Therefore, there is difficulty in application to a copper foil as an external layer of a printed wiring board.
On the other hand, a process for providing a resin layer with a high laser light absorption efficiency on a surface of a copper foil is a process that a film constituting a resin layer is laminated on a surface of a copper clad laminate or a process that a liquid resin composition is applied on the surface of a copper clad laminate and cured. In the case of adoption of the laminating process, a necessity arises for introduction of a high precision apparatus for stable laminating, unpreferably resulting in cost increase. In the case where a resin composition is applied in a liquid state to thereafter cure as well, a necessity arises for facilities such as a curing oven and others, which requires a significant capital investment, thereby pushing up a product cost as a result. Furthermore, a material constituting a resin layer is an organic agent and almost cannot be recycled, which cannot be said to be preferable from the viewpoint of protection of the natural environment.